1. Field of the Invention
This invention relates to a method for manufacturing photovoltaic modules, in particular, relates to a method for manufacturing a photovoltaic module in which a plurality of solar cells are electrically connected to each other by connecting members referred to as connecting tabs, connecting leads, or the like.
2. Description of Related Art
With the recent growing expectation on clean energy to solve the global environment conservation issues such as the global warming, attention is being given to solar batteries, as a clean energy source, which directly convert sunlight into electric energy. Typical photovoltaic devices using these solar batteries for power generation include a plurality of solar cells and are configured to electrically connect the adjacent solar cells to each other in series or in parallel by soldering with connecting tabs, each made of a copper film, in order to obtain a desired output.
In Japanese unexamined patent publication No. 2003-168811, these connecting tabs are used as lead wires for connecting the plurality of solar cells in series or in parallel and are also used as output terminals.
A description will be made on a typical interconnection process in which the connecting tabs are connected to solar cells. Firstly, a plurality of solar cells each having bus bar electrodes provided on their surfaces for the tab connection are prepared (the first step). Next, the plurality of solar cells are applied with flux (the second step), subsequently a connecting tab is disposed from an upper surface of a solar cell to a lower surface of an adjacent solar cell by using solder (the third step). Then, the disposed connecting tab is pressed from above against the cell so that the connecting tab does not lift up, while being heated to solder the connecting tab (the fourth step).
The interconnecting processes currently in vogue fall into two broad categories based on the heating method for connecting the connecting tab: a lamp heating interconnection scheme; and a hot-air heating interconnection scheme.
In the lamp heating interconnection scheme, light from a light source that is a lamp such as a halogen lamp is focused and irradiated onto the connecting tab to be soldered. The light irradiation from the lamp with an output of approximately from 1500 W to 2500 W for a few seconds enables the soldering of the interconnector with the use of lead-free solder. This scheme has an advantage of a relatively quick temperature rise; however, exact heat control is difficult, therefore causing more cracks or warpage in the cells. On the other hand, in the hot-air heating interconnection scheme, hot air in a temperature range of 250 degrees Celsius (° C.) to 480 degrees Celsius but equal to the solder's melting temperature or higher is applied to the proximity of the connecting tab. The soldering of the interconnector with lead-free solder can be achieved by applying hot air having a temperature equal to the solder's melting temperature or higher for a few seconds. The hot-air heating interconnection scheme involves high accurate heat control, thereby preventing cracks and warpage in the cells and improving yields.
The photovoltaic module in which the connecting tabs are connected through the interconnecting process may have residual components of flux and organic substances used during the interconnecting process. In order to remove the residues without cleaning, International publication No. WO 2005/096396 A1 proposes a method for evaporating flux by heating the cells with the tabs connected thereto.
A solar cell includes several junctions between a surface collector electrode and a semiconductor layer and between a surface anti-reflection layer and an electrode layer and interface junctions between semiconductor layers. It is known that a cell's output is improved by enhancing the interface properties. Generally, hydrogen passivation or thermal annealing is often performed.
There has been a great deal of research and commercialization of solar batteries using crystalline semiconductors such as single-crystal silicon and polycrystalline silicon. Noteworthy among these is a solar cell having a semiconductor heterojunction formed by combining amorphous silicon and crystalline silicon since the junction can be formed through a low temperature process, 200 degrees Celsius or lower, such as a plasma CVD method, and such a solar cell having the junction can provide high conversion efficiency. In order to improve the photoelectric conversion efficiency of this type of solar cell, it is necessary to improve fill factor (F.F.), while maintaining high short-circuit current (Isc) and open-circuit voltage (Voc).
To this end, there developed a solar cell in which a substantially intrinsic amorphous silicon layer (i-type amorphous silicon layer) containing hydrogen is interposed between an n-type single-crystal silicon substrate and a p-type amorphous silicon layer containing hydrogen.
However, the solar cell having the amorphous silicon layer formed through the CVD method have the output characteristics deteriorate if heated for a long time at a temperature as high as 200 degrees Celsius or more.
The possible causes of the deterioration in the output characteristics by heating at high temperatures are: (1) diffusion of electrode materials into doped amorphous semiconductor thin films containing hydrogen; (2) diffusion of the dopant into substantially intrinsic amorphous semiconductor thin-film layers; (3) diffusion of hydrogen; and others. Among these, cause (3) affects the output characteristics the most at low temperatures.
It is known, as discussed above, that thermal annealing is effective in improving the output characteristics. However, the solar cell having the amorphous silicon layer formed through the CVD method may have the output characteristics deteriorate due to being heated for a long time at high temperatures. Because of this, a forming process of the electrodes, a laminating process and so forth that are performed after formation of the amorphous silicon layer are so controlled not to exceed the temperature adopted in the CVD process. However, further improvement is desired for the output characteristics of such formed solar cells.
Since the object of WO 2005/096396 A1 is to vaporize the flux residues, the solar cell is exposed to heat that is higher than the temperature for vaporizing flux after soldering. For this reason, there is no adequate consideration for the heat treatment for improving the output characteristics of the solar cells.